The present invention relates to a display device having a gamma correction circuit and a method for driving the same. Particularly, the invention relates to a light emitting device having light emitting elements disposed on an insulating surface for displaying images with the light emitting elements and a method for driving the same. Additionally, the invention relates to an electronic device provided with the display device.
In recent years, the development of display devices for displaying images has been proceeding. In the display devices, there are a liquid crystal display device for displaying images with liquid crystal elements and a light emitting device for displaying images with light emitting elements such as an Organic Light Emitting Diode (OLED).
The gray scales of brightness of images to be displayed on the display device are expressed by luminance. Furthermore, the gray scales are used for two meanings: the real luminance scale defined by using the luminance as physical quantity expressed by a unit of cd/m2 as a standard for equal split, and the perceived luminance scale defined by using the human visible perception property as a standard for equal split. Moreover, the gray scales are sometimes denoted by bit. In this case, for example, one bit expresses 21=two levels of gray scale, three bits expresses 23=eight levels of gray scale, and n bits expresses 2n levels of gray scale.
FIGS. 12A to 12C are used to describe the relationship between the gray scales and the luminance. FIG. 12A depicts a graph in which the horizontal axis is the real luminance scale (XR) and the vertical axis is the real luminance (YR). The real luminance scale (XR) is directly proportional to the real luminance (YR), being a linear line ever increasing. The higher the real luminance scale (XR) rises, the greater the real luminance (YR) becomes. In other words, the brightness becomes intense as the real luminance scale (XR) rises higher.
FIG. 12B depicts a graph in which the horizontal axis is the real luminance scale (XR) and the vertical axis is the perceived luminance (YH). In addition, FIG. 12C depicts a graph in which the horizontal axis is the perceived luminance scale (XH) and the vertical axis is the real luminance (YR). As shown in FIGS. 12B and 12C, the visibility to the brightness of human becomes dull as the luminance becomes higher. Consequently, the gray scales are expressed as a curve as shown in FIG. 12B to human eyes, not the linear line directly proportional to the luminance.
Here, suppose the video signal (X) to be inputted to a display device is a type that is proportional to the perceived luminance. Then, suppose the display device is a type that performs the output (Y) of real luminance type. In this case, the display device is desired first to convert the video signal (X) into data type proportional to the real luminance. When the following Equation (1) is used for the conversion at this time, this conversion is called gamma correction. So-called inverse gamma correction and strict gamma correction are also called gamma correction, not specified particularly.Y=Xγ  (1)
Generally, when the display device such as a liquid crystal display device and a light emitting device is used to display images, a gamma correction circuit for the gamma correction is disposed to truly reproduce the images to human eyes. Gamma correction allows the relationship between the gray scale (X) of the video signal to be inputted to a display device and the luminance (Y) to be outputted (displayed) by the display device to be corrected to the optimal curve. Suppose X is the perceived luminance type and Y is the real luminance type, it is set to gamma=about 2.2 in general.
Here, the light emitting device for displaying images with the light emitting elements will be described in detail. In addition, the light emitting device is roughly classified into the passive type and the active type. The active light emitting device having a light emitting element and a TFT for controlling the light emitting element both disposed in each of pixels on a substrate will be described in detail.
As a driving method when images of multiple gray scales are displayed in the active light emitting device, an analogue gray scale system and a digital gray scale system are named. The difference between both systems is in the method for controlling light emitting elements in the states of the light emitting element to emit light and not to emit light. The analogue gray scale system is the system that controls the current carried thorough the light emitting element to obtain gray scales. The digital gray scale system is the system that the light emitting element is driven only by two states, the ON-state (the state that the luminance is nearly 100%) and the OFF-state (the state that the luminance is nearly 0%). However, the digital gray scale system can display only two levels of gray scale if nothing is done, thus being combined with another system to display the images of multiple levels of gray scale.
One system is that the digital gray scale system is combined with the area gray scale system. The area gray scale system is the system that a single pixel is split into a plurality of sub-pixels and each of the sub-pixels is controlled to emit light or not to emit light, whereby the gray scales are displayed by the total area emitting light inside the single pixel.
Another system is that the digital gray scale system is combined with the time gray scale system. The time gray scale system is the system that time for a light emitting element to emit light (time for a pixel to be lighted) is controlled to display the gray scales. More specifically, suppose a period of time for drawing images one time is one frame period. The one frame period is split into a plurality of subframe periods having different length to select the light emitting element to emit light or not to emit light during each of the subframe periods, whereby the difference in the length of time emitted within one frame period expresses the gray scales.
For example, when an image of eight levels of gray scale (equivalent to three bits) is displayed in the real luminance, one frame period is split into three subframe periods SF1 to SF3. Then, light emission or no light emission is selected in each of the subframe periods SF1 to SF3, whereby the lengths of the total light emitting time are utilized to express the eight levels of gray scales, 0%, 14%, 28%, 43%, 57%, 71%, 86% and 100% luminance. For example, when light emission is selected only in the subframe period SF1 and no light emission is selected in the other subframe period SF2 and SF3, the luminance is 57%. Additionally, when light emission is selected in the subframe periods SF1 and SF3 and no light emission is selected in the other subframe period SF2, the luminance is 71%. In this manner, the time gray scale system expresses the gray scales by the combination of light emission time expressed in 2k unit. Furthermore, k is expressed in the range of 0≦k≦n−1, and 2n expresses the greatest gray scale allowing display in a light emitting device.
However, the gray scales might have omissions in gamma correction in the light emitting device in which the digital gray scale system is adapted to express the gray scales by combining with the area gray scale system in 2k unit (binary code), or to express the gray scales by combining with the time gray scale system. Here, the omission in gray scales will be described by FIGS. 15A and 15B.
FIG. 15A depicts a graph in which the horizontal axis is the real luminance scale (XR) and the vertical axis is the perceived luminance (YH). FIG. 15B depicts a graph in which the horizontal axis is the perceived luminance scale (XR) and the vertical axis is perceived luminance (YH).
The graphs shown in FIGS. 15A and 15B reveal that the changes in the perceived luminance corresponding to the levels of real luminance scale are generally gradual when the levels of real luminance scale are high. Additionally, when the levels of real luminance scale are low, the changes in the perceived luminance corresponding to the levels of real luminance scale become greater suddenly.
That is, since the low perceived luminance area has fewer levels of real luminance scale than the high perceived luminance area has, the lower perceived luminance area tends to have greater errors. Then, the levels of real luminance scale corresponding to the perceived luminance might not exist in the low perceived luminance area, and thus some gray scales cannot be expressed. In this manner, when the areas of real luminance scale corresponding to the perceived luminance do not exist, omissions are generated in the gray scales.